The singer -- who goes by Aza -- is filing the lawsut in L.A., claiming "Call Me Maybe" is a carbon copy of her song "Hunky Santa" ... which producers simply tweaked before adding Carly Rae Jepsen's voice.

According to the lawsuit -- to be filed today by Aza's lawyer Barry Rothman -- she's suing for unspecified damages ... but based on how insanely popular "Call Me Maybe" is, a ton of money's on the line.

Aza tells us, "I'm shocked and surprised that these people wanted to sample my lyrics on their song. They didn't ask me for permission, they just took it. That's why I filed this lawsuit. When I first heard it on the radio, I was driving and almost got into an accident. I couldn't believe what I was hearing."

As for how similar the songs REALLY are -- you be the judge.

Scooter Braun -- Justin Bieber's manager -- is also named in the suit.

Carly's rep tells TMZ, "This is completely false and [Carly's] lawyers will deal with this. Everyone knows [Carly] is a songwriter. She is not spending a lot of time listening to Ukrainian radio."